Measurements of precipitating particles and the electric field on board EXOS D spacecraft for the period January--June 1990 have been used for estimation of the diameter of the polar cap along dawn-dusk meridian and the polar cap voltage. Identification of the polar cap boundaries has been made on the basis of specific features of precipitating ions. The data on the polar cap boundary location obtained for different geophysical conditions have been used to derive the statistical relationship between the polar cap diameter and PC index. The analysis has shown an approximately linear relationship between the polar cap diameter and the PC index for values PC<3, the diameter tending to be asymptote when the PC index reaches large positive values. Cross polar cap voltage derived from EXOS D data is in good correlation with interplanetary quantities including the interplanetary magnetic field (IMF) southward component. The best correlation is obtained for the merging electric field vBT sin2 &thgr;/2, with a coefficient of correlation higher than 0.82. Almost the same correlation is observed between polar cap voltage and PC index. The effect of ''saturation'' is not traced in the voltage dependencies on the PC index and interplanetary quantities up to values BT≤10 nT. ¿ American Geophysical Union 1996